1. Technical Field
This invention relates to an active noise absorption system to reduce aircraft engine noise. Specifically, the invention relates to a system having an active acoustic liner on interior engine surfaces and related control elements that absorb noise generated by the fans and turbines of modern aircraft engines.
2. Background Art
There is a great need to reduce the noise levels generated by commercial and military aircraft at ground levels near runways. One current solution is to use passive acoustic liners with fixed geometry in the engine inlet surfaces. Such acoustic liners consist of a honeycomb core that is covered by a porous face sheet. Each of the cells of the honeycomb acts as a Helmholtz resonator to absorb acoustic energy. The cells will absorb a maximum amount of incoming acoustic energy only at the resonant frequency of the cell, which absorption decreases as the incoming acoustic energy changes from the resonant frequency. The size and depth of the honeycomb cells and the porosity of the face sheet effect the noise absorption characteristics of the liner.
This type of passive honeycomb liner will not, however, meet the quickly-increasing noise requirements imposed on such engines by local authorities and the Federal Aviation Administration. In fact, many aircraft will be forced out of service prior to their planned service life if engine noise levels cannot be reduced in an efficient and economic manner. For example, some noise reduction methods such as hush kits provide effective noise level reduction, but are expensive and add weight to the aircraft. The added weight degrades engine performance and reduces fuel economy.
Actively controlling the conditions inside the honeycomb cell provides many advantages. The structure of a passive acoustic liner are usually designed to optimize noise absorption in a narrow frequency range of their resonant frequency, such as a frequency related to the angular velocity of the engine and the number of turbine blades. A typical target frequency of noise to be absorbed is approximately 1,000 Hz. However, the predominant frequency of noise to be absorbed changes with particular flight conditions of the aircraft, for example during take off or airport approach. By controlling the conditions inside the cell, however, the optimum noise absorption performance can be maintained over a wide range of flight conditions and frequencies.
One problem with active acoustic liners that have been proposed is that current designs have not provided a practical solution. For example, one approach has been to generate cancelling noise fields generated with acoustical inputs, i.e., out of phase signals with equal amplitudes. One implementation of this approach has been to place speakers behind or in the cells of the acoustic liner. The added size and weight of such systems, however, has made them impractical. Further, such systems are not robust and consume substantial power. In addition, if a speaker is required for each honeycomb cell, numerous speakers would be required adding to the expense and reliability of the system.
Thus, it is one object of the invention to provide an active acoustic liner that is light in weight and small in size. These objects will minimize the effects on engine and aircraft performance of the system.
Another object of the invention is to provide an active acoustic liner that is rugged and able to withstand the severe shock, vibration and temperature present in the engine inlets.
Another object of the invention is to provide an active acoustic liner with few active components to increase its time between failures and simplify maintenance of the liner system.
Another object of the invention is to provide an active acoustic liner system that can be used with existing passive liner designs. This object will reduce implementation costs and qualification time.